What your BMR includes
Your basal metabolic rate is the energy used to keep you alive at complete rest: breathing, circulating blood, running your brain, liver, kidneys and heart, and keeping your temperature steady. Strictly it is measured first thing in the morning, fasted, lying still in a warm room. The same measurement taken under looser conditions is called resting metabolic rate (RMR), and it is usually a little higher. Calculators use the two terms almost interchangeably.
For most adults BMR is the biggest part of daily calorie use, commonly somewhere around 60 to 70 percent of the total. It is the starting point for the TDEE calculator, which adds movement and digestion on top.
The three equations
The headline number uses Mifflin–St Jeor, published in 1990 and widely used by dietitians. The results table also shows the revised Harris–Benedict equation (1984) and, if you enter your body fat, Katch–McArdle.
| Equation | Formula |
|---|---|
| Mifflin–St Jeor, male | 10 × weight + 6.25 × height − 5 × age + 5 |
| Mifflin–St Jeor, female | 10 × weight + 6.25 × height − 5 × age − 161 |
| Revised Harris–Benedict, male | 88.362 + 13.397 × weight + 4.799 × height − 5.677 × age |
| Revised Harris–Benedict, female | 447.593 + 9.247 × weight + 3.098 × height − 4.330 × age |
| Katch–McArdle | 370 + 21.6 × lean body mass (kg), where lean mass = weight × (1 − body fat %) |
Calories convert to kilojoules at 4.184 kJ per kcal, which is why the result also shows kJ.
Worked example
A 41-year-old woman, 160 cm tall, weighing 58 kg, with about 28% body fat.
- Mifflin–St Jeor: 10 × 58 + 6.25 × 160 − 5 × 41 − 161 = 1,214 kcal (5,079 kJ)
- Revised Harris–Benedict: 447.593 + 9.247 × 58 + 3.098 × 160 − 4.330 × 41 = 1,302 kcal
- Katch–McArdle: lean mass = 58 × (1 − 0.28) = 41.8 kg, so 370 + 21.6 × 41.8 = 1,272 kcal
The three answers span 88 calories. That spread is typical and is why no BMR figure should be treated as exact.
How BMR changes with age
Mifflin–St Jeor takes off 5 calories a day for every year of age. Here is what that does for the woman in the example if only her age were different:
| Age | BMR (kcal a day) |
|---|---|
| 20 | 1,319 |
| 30 | 1,269 |
| 40 | 1,219 |
| 50 | 1,169 |
| 60 | 1,119 |
| 70 | 1,069 |
The equation builds in an average decline. In real life much of the drop comes from losing muscle, and muscle is something you can keep with strength training and enough protein. As a rough guide, each kilogram of extra muscle raises resting burn by only about 13 calories a day, so the benefit of training shows up mostly in what you burn while moving.
Which equation should you trust?
- Mifflin–St Jeor is a sensible default for most adults who do not know their body fat.
- Harris–Benedict is older and tends to give a slightly higher figure, as in the example.
- Katch–McArdle can suit lean or muscular people because it uses lean mass, but only if your body fat estimate is reliable. A guess that is 10 percentage points off moves the result by roughly 125 calories for a person of this size.
The only accurate way to know your BMR is a lab test called indirect calorimetry, which measures the oxygen you use. If you are not planning that, use the calculators as a starting point and adjust by watching your weight over a few weeks.
These results are general estimates for healthy adults, not medical advice. See the disclaimer for who should check with a professional first.